Added JSON Content-Type header to freemobile://

[caronc]

Description:

Related issue (if applicable): #1090

Added Content-Type of application/json to service submission.

Checklist

• [ ] The code change is tested and works locally.
• [x] There is no commented out code in this PR.
• [x] No lint errors (use flake8)
• [x] 100% test coverage

Testing

Anyone can help test this source code as follows:

# Create a virtual environment to work in as follows:
python3 -m venv apprise

# Change into our new directory
cd apprise

# Activate our virtual environment
source bin/activate

# Install the branch
pip install git+https://github.com/caronc/apprise.git@1090-free-mobile-update-2

# Test out the changes with the following command:
apprise -t "Test Title" -b "Test Message" \
  freemobile://credentials

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.27%. Comparing base (50c1a0d) to head (f79871c).

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1109 +/- ## ======================================= Coverage 99.27% 99.27% ======================================= Files 139 139 Lines 18053 18053 Branches 3688 3688 ======================================= Hits 17923 17923 Misses 121 121 Partials 9 9 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.