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##### SUMMARY
<!--- Explain the problem briefly below -->
in postgresql_set.py:303 the value to be set is converted to uppercase if it contains "mb" or "gb" or "tb".
for example an archive command will fail if the case is not correct.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
postgresql_set.py:303
##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.8.1
python version = 2.7.13 (default, Sep 26 2018, 18:42:22) [GCC 6.3.0 20170516]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->
debian stretch

##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml
    postgresql_set:
      name: 'archive_command'
      value: 'test ! -f /mnt/postgres/mb/%f && cp %p /mnt/postgres/mb/%f'
```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
grep archive_command postgresql.auto.conf:
```
archive_command = 'test ! -f /mnt/postgres/mb/%f && cp %p /mnt/postgres/mb/%f'
```

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->
grep archive_command postgresql.auto.conf:
```
archive_command = 'TEST ! -F /MNT/POSTGRES/MB/%F && CP %P /MNT/POSTGRES/MB/%F'
```

<!--- Paste verbatim command output between quotes -->
```paste below

```

restart_required=False,
            value_pretty="",
            prev_val_pretty="",
            value={"value": "", "unit": ""},
        )
        kw['name'] = name
        db_connection.close()
        module.exit_json(**kw)
    
    restart_required = False
    changed = False
    kw['name'] = name
    kw['restart_required'] = False
    
    res = param_get(cursor, module, name)
    current_value = res[0]
    raw_val = res[1]
    unit = res[2]
    boot_val = res[3]
    context = res[4]
    if value == 'True':
        value = 'on'
    elif value == 'False':
        value = 'off'
    kw['prev_val_pretty'] = current_value
    kw['value_pretty'] = deepcopy(kw['prev_val_pretty'])
    kw['context'] = context
    
    if context == "internal":
        module.fail_json(msg="%s: cannot be changed (internal context). See "