fix: FirstLineContainsHeader type from int to bool, update respective test cases

[jameskim0987]

Fixes #61

Changes made in this PR:

1. change type of FirstLineContainsHeader from int to bool
2. update respective testcases ( TestTranslationMemoryService_ImportTM and TestTranslationMemoryService_CheckTMImportStatus ) to use and expect bool instead of int

[jameskim0987]

Hi @andrii-bodnar @Multiply, this PR is ready for review

[andrii-bodnar]

@jameskim0987 thanks a lot for the contribution!

[vorobeyme]

Thanks for the contribution!
I just wanted to flag up the data.attributes.firstLineContainsHeader parameter. According to the API doc, it must be integer.

@andrii-bodnar, could you please confirm that it is always bool, int, or both?
If it's both, then we'll need to specify custom decoding from JSON to support all of them.

[andrii-bodnar]

@vorobeyme it's some mistake in the API Docs (I see only one wrong integer example in the Check TM Import Status method). The firstLineContainsHeader is always boolean.