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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

```

description: Complete reference of the pg_settings view documentation.
  link: https://www.postgresql.org/docs/current/view-pg-settings.html
- name: PostgreSQL ALTER SYSTEM command reference
  description: Complete reference of the ALTER SYSTEM command documentation.
  link: https://www.postgresql.org/docs/current/sql-altersystem.html
author:
- Andrew Klychkov (@Andersson007)
extends_documentation_fragment: postgres
'''
EXAMPLES = r'''
- name: Restore wal_keep_segments parameter to initial state
  postgresql_set:
    name: wal_keep_segments
    reset: yes
# Set work_mem parameter to 32MB and show what's been changed and restart is required or not
# (output example: "msg": "work_mem 4MB >> 64MB restart_req: False")
- name: Set work mem parameter
  postgresql_set:
    name: work_mem
    value: 32mb
    register: set
- debug:
    msg: "{{ set.name }} {{ set.prev_val_pretty }} >> {{ set.value_pretty }} restart_req: {{ set.restart_required }}"
  when: set.changed
# Ensure that the restart of PostgreSQL server must be required for some parameters.
# In this situation you see the same parameter in prev_val and value_prettyue, but 'changed=True'
# (If you passed the value that was different from the current server setting).
- name: Set log_min_duration_statement parameter to 1 second
  postgresql_set:
    name: log_min_duration_statement