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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

```

default: false
  session_role:
    description:
    - Switch to session_role after connecting. The specified session_role must
      be a role that the current login_user is a member of.
    - Permissions checking for SQL commands is carried out as though
      the session_role were the one that had logged in originally.
    type: str
  db:
    description:
    - Name of database to connect.
    type: str
    aliases:
    - login_db
notes:
- Supported version of PostgreSQL is 9.4 and later.
- Pay attention, change setting with 'postmaster' context can return changed is true
  when actually nothing changes because the same value may be presented in
  several different form, for example, 1024MB, 1GB, etc. However in pg_settings
  system view it can be defined like 131072 number of 8kB pages.
  The final check of the parameter value cannot compare it because the server was
  not restarted and the value in pg_settings is not updated yet.
- For some parameters restart of PostgreSQL server is required.
  See official documentation U(https://www.postgresql.org/docs/current/view-pg-settings.html).
seealso:
- module: postgresql_info
- name: PostgreSQL server configuration
  description: General information about PostgreSQL server configuration.
  link: https://www.postgresql.org/docs/current/runtime-config.html
- name: PostgreSQL view pg_settings reference