Resolve #172. Replaced String HTTP method with Enum

[sceiler]

Description

This resolves #172. The change will better prevent typos because it forces the use of an enum when using HTTP methods.

Motivation and Context

#172

How Has This Been Tested?

Ran unit tests.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (change which improves current code base; please describe the change)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Screenshots (if appropriate):

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• All new and existing tests passed locally
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov-commenter]

Codecov Report

Merging #175 (7d5a270) into 1.1.0_dev (b4c8390) will increase coverage by 0.69%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##             1.1.0_dev     #175      +/-   ##
===============================================
+ Coverage        58.89%   59.59%   +0.69%     
- Complexity         132      134       +2     
===============================================
  Files                8        9       +1     
  Lines              635      646      +11     
  Branches            44       44              
===============================================
+ Hits               374      385      +11     
  Misses             232      232              
  Partials            29       29              

Flag	Coverage Δ
unittests	59.59% <100.00%> (+0.69%)	⬆️

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Impacted Files	Coverage Δ
.../main/java/com/saucelabs/saucerest/HttpMethod.java	100.00% <100.00%> (ø)
...c/main/java/com/saucelabs/saucerest/SauceREST.java	66.88% <100.00%> (ø)

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[diemol]

Thank you, @sceiler!