[discovery] Add update_document method

[maniax89]

Adds in the https://watson-api-explorer.mybluemix.net/apis/discovery-v1#!/Documents/updateDocument method for Discovery that was missing. Required in order to have control over document ID when uploading (as well as being able to change an existing document without 2 steps of delete + add)

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #233 into master will decrease coverage by 2.89%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master     #233     +/-   ##
=========================================
- Coverage   84.31%   81.42%   -2.9%     
=========================================
  Files          24       24             
  Lines        1167     1179     +12     
=========================================
- Hits          984      960     -24     
- Misses        183      219     +36

Impacted Files	Coverage Δ
watson_developer_cloud/discovery_v1.py	97.63% <75%> (-2.37%)	⬇️
watson_developer_cloud/language_translator_v2.py	44.82% <0%> (-31.04%)	⬇️
..._developer_cloud/watson_developer_cloud_service.py	63.54% <0%> (-9.38%)	⬇️
watson_developer_cloud/tone_analyzer_v3.py	92.3% <0%> (-7.7%)	⬇️
watson_developer_cloud/dialog_v1.py	38.88% <0%> (-5.56%)	⬇️
watson_developer_cloud/document_conversion_v1.py	93.75% <0%> (-3.13%)	⬇️

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