generalize args for NLU

[shuisman]

At the moment the args are fixed, which is not scalable. In line with other endpoints, this needs to be generalized.

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #250 into master will decrease coverage by 2.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage   85.23%   82.44%   -2.79%     
==========================================
  Files          24       24              
  Lines        1219     1219              
==========================================
- Hits         1039     1005      -34     
- Misses        180      214      +34

Impacted Files	Coverage Δ
...veloper_cloud/natural_language_understanding_v1.py	100% <ø> (ø)	⬆️
watson_developer_cloud/language_translator_v2.py	44.82% <0%> (-31.04%)	⬇️
..._developer_cloud/watson_developer_cloud_service.py	66.66% <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%)	⬇️
watson_developer_cloud/visual_recognition_v3.py	98.73% <0%> (-1.27%)	⬇️

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[germanattanasio]

@shuisman even if you make the arguments dynamic **kwargs they won't do anything because you are not using them in WatsonDeveloperCloudService

[shuisman]

@germanattanasio, This is true. But at the moment the arguments are fixed. So for each new introduced functionality you have to add them here as well at the moment. With this change makes it future proof and in line with the other endpoints.
In example, x_watson_learning_opt_out is not available at NLU at the moment.

[germanattanasio]

@shuisman new functionality doesn't actually change the constructor but If you want to do this you will have to do it for all the service. Right now you are only changing NLU

[jsstylos]

We used to only support x_watson_learning_opt_out for Speech, but now I think more services are using it, so it's reasonable to use **kwargs to add it for the services that need it.

[germanattanasio]

If we are going to support **kwargs I would prefer to support default headers. Like in the Node SDK

[jsstylos]

We should add support for custom headers, and then switch the rest of the services to use **kwargs to support them

[shuisman]

can this PR be merged please?