Bitbucket: fix merge_pull_request. POSTparams --> data

[jorgegomzar]

Hello there 🖖

I was testing this in my local and Bitbucket Cloud was raising an error code whenever I tried to merge a PR.
Wanted to maintain previous version and did not notice that params dict was being sent as URL encoded params. The new params should be a JSON encoded body for the POST request instead --> source

I've fixed it and tested with my own repositories in Bitbucket Cloud 👍

QA:

doc8: commands[0]> doc8 --ignore-path docs/_build/ docs/
Scanning...
Validating...
========
Total files scanned = 9
Total files ignored = 0
Total accumulated errors = 0
Detailed error counts:
    - doc8.checks.CheckCarriageReturn = 0
    - doc8.checks.CheckIndentationNoTab = 0
    - doc8.checks.CheckMaxLineLength = 0
    - doc8.checks.CheckNewlineEndOfFile = 0
    - doc8.checks.CheckTrailingWhitespace = 0
    - doc8.checks.CheckValidity = 0
  py3: OK (9.81=setup[5.23]+cmd[0.14,2.34,2.10] seconds)
  flake8: OK (1.48=setup[0.01]+cmd[1.47] seconds)
  black: OK (2.16=setup[0.01]+cmd[2.15] seconds)
  mypy: OK (0.85=setup[0.01]+cmd[0.84] seconds)
  bandit: OK (1.83=setup[0.01]+cmd[1.82] seconds)
  doc8: OK (0.86=setup[0.01]+cmd[0.85] seconds)
  congratulations :) (17.14 seconds)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 34.42%. Comparing base (b76bcea) to head (70c0da4).
Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
atlassian/bitbucket/__init__.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1438      +/-   ##
==========================================
- Coverage   34.42%   34.42%   -0.01%     
==========================================
  Files          46       46              
  Lines        8592     8593       +1     
  Branches     1602     1602              
==========================================
  Hits         2958     2958              
- Misses       5511     5512       +1     
  Partials      123      123              

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