add toDictionary, toJSONString

[iamchiwon]

My project using Firebase, needs convertings between Model and Dictionary, [String:Any]. So I added "toDictionary()" and "toJSONString()" for POST request

1. when error occured, toDictionay() and toJSONString() returns empty not nil , followed your map() that returns empty Data when error
2. respected your coding convention, 2 space indent, and space after colon

[codecov-io]

Codecov Report

Merging #2 into master will decrease coverage by 4.87%. The diff coverage is 83.56%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
- Coverage    92.5%   87.62%   -4.88%     
==========================================
  Files           4        3       -1     
  Lines          40       97      +57     
  Branches        3        0       -3     
==========================================
+ Hits           37       85      +48     
- Misses          0       12      +12     
+ Partials        3        0       -3

Impacted Files	Coverage Δ
Sources/RxCodable/ObservableType+RxCodable.swift	87.09% <82.6%> (-3.82%)	:arrow_down:
Sources/RxCodable/Single+RxCodable.swift	87.87% <84%> (-3.04%)	:arrow_down:
Sources/RxCodable/Maybe+RxCodable.swift	87.87% <84%> (-3.04%)	:arrow_down:
Sources/TestUtil/Fixture.swift

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[iamchiwon]

1. toDictionary(), toJSONString() got parameter JSONEncoder as first with no name.
2. toDoctionary() throws RxError.noElements. RxError.noElements seems not exactly matched, but I don't want to create new ErrorType
3. UnitTest added, that shows [Encodable] ==> JSON string.