**Base calculation amount is wrong when multiple taxes**

Impacted versions:
- 9.0 Enterprise (code from 2016/01/18)

Steps to reproduce:
I need to be able to have multiple taxes as followed:
- Customer Price 1000
- Base calculation for VAT $ 870 (reversed tax)
- IVA 13.00%    21402 DÉBITO FISCAL    $ 130.00
- IT 3% 2140106 Impuesto a las Transacciones (Calculated on Price+VAT)  $ 30.00
- IT2 3%    5310901 Impuesto a las Transacciones    (Calculated on Price+VAT) $ -30.00

Current behavior:
Setup VAT:
![1vat setup](https://cloud.githubusercontent.com/assets/7600613/12437866/abb6b6de-bf5c-11e5-9b6e-85247914c7ab.png)
![1vat setup2](https://cloud.githubusercontent.com/assets/7600613/12437868/ae64eb6c-bf5c-11e5-9124-fd58de0d4af5.png)
SO is correct only using the VAT:
![2so only vat correct](https://cloud.githubusercontent.com/assets/7600613/12437882/beee11ca-bf5c-11e5-8430-016ad23c3e04.png)
IT setup:
![3tax group setup](https://cloud.githubusercontent.com/assets/7600613/12437887/c9025e1e-bf5c-11e5-8176-1c47fd6bcf62.png)
![4it1 setup1](https://cloud.githubusercontent.com/assets/7600613/12437888/ca8dc2c8-bf5c-11e5-8f32-ca7fbfbbcd15.png)
![4it1 setup2](https://cloud.githubusercontent.com/assets/7600613/12437889/cc3418c0-bf5c-11e5-9f09-58660a3595e9.png)
![4it2 setup1](https://cloud.githubusercontent.com/assets/7600613/12437892/cdf5d8ba-bf5c-11e5-8b6b-168a54fc9de5.png)
![4it2 setup2](https://cloud.githubusercontent.com/assets/7600613/12437893/cf7583fc-bf5c-11e5-8db5-32efe0f11a47.png)

Besides the tax group sequence is
- impuestos =0
- IT = 20

Now adding the IT as second 
![5so vat and it incorrect](https://cloud.githubusercontent.com/assets/7600613/12437912/f10eb10a-bf5c-11e5-949f-4b73dce1fc70.png)
Invoice is incorrect as well:
![6invoice1](https://cloud.githubusercontent.com/assets/7600613/12437916/fce2013a-bf5c-11e5-9870-e015bd56106a.png)
![6invoice2](https://cloud.githubusercontent.com/assets/7600613/12437919/fefff9e0-bf5c-11e5-92ec-1990bf87be1a.png)

Finally accounting Entries:
![7accounting entry](https://cloud.githubusercontent.com/assets/7600613/12437921/07523dce-bf5d-11e5-939e-b00a6cf1e0c8.png)

Expected behavior:
Taxes amount are properly calculated but not base calculation. 
in SO, invoice and accounting entry the base calculation is wrong (1000 instead of 870 expected)
It seems that the value is overidden when the second tax is computed.

addons/account/models/account.py
'amount': tax_amount,
                'base': tax_base,
                'sequence': tax.sequence,
                'account_id': tax.account_id.id,
                'refund_account_id': tax.refund_account_id.id,
                'analytic': tax.analytic,
            })
        return {
            'taxes': sorted(taxes, key=lambda k: k['sequence']),
            'total_excluded': currency.round(total_excluded) if round_total else total_excluded,
            'total_included': currency.round(total_included) if round_total else total_included,
            'base': base,
        }
    @api.v7
    def compute_all(self, cr, uid, ids, price_unit, currency_id=None, quantity=1.0, product_id=None, partner_id=None, context=None):
        currency = currency_id and self.pool.get('res.currency').browse(cr, uid, currency_id, context=context) or None
        product = product_id and self.pool.get('product.product').browse(cr, uid, product_id, context=context) or None
        partner = partner_id and self.pool.get('res.partner').browse(cr, uid, partner_id, context=context) or None
        ids = isinstance(ids, (int, long)) and [ids] or ids
        recs = self.browse(cr, uid, ids, context=context)
        return AccountTax.compute_all(recs, price_unit, currency, quantity, product, partner)
    @api.model
    def _fix_tax_included_price(self, price, prod_taxes, line_taxes):
        """Subtract tax amount from price when corresponding "price included" taxes do not apply"""
        
        incl_tax = prod_taxes.filtered(lambda tax: tax not in line_taxes and tax.price_include)
        if incl_tax:
            return incl_tax.compute_all(price)['total_excluded']
        return price
class AccountOperationTemplate(models.Model):