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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

```

short_description: Change a PostgreSQL server configuration parameter
description:
   - Allows to change a PostgreSQL server configuration parameter.
   - The module uses ALTER SYSTEM command and applies changes by reload server configuration.
   - ALTER SYSTEM is used for changing server configuration parameters across the entire database cluster.
   - It can be more convenient and safe than the traditional method of manually editing the postgresql.conf file.
   - ALTER SYSTEM writes the given parameter setting to the $PGDATA/postgresql.auto.conf file,
     which is read in addition to postgresql.conf.
   - The module allows to reset parameter to boot_val (cluster initial value) by I(reset=yes) or remove parameter
     string from postgresql.auto.conf and reload I(value=default) (for settings with postmaster context restart is required).
   - After change you can see in the ansible output the previous and
     the new parameter value and other information using returned values and M(debug) module.
version_added: '2.8'
options:
  name:
    description:
    - Name of PostgreSQL server parameter.
    type: str
    required: true
  value:
    description:
    - Parameter value to set.
    - To remove parameter string from postgresql.auto.conf and
      reload the server configuration you must pass I(value=default).
      With I(value=default) the playbook always returns changed is true.
    type: str
  reset:
    description:
    - Restore parameter to initial state (boot_val). Mutually exclusive with I(value).
    type: bool