Handle numeric strings translation values properly

[andrius-spotos]

Summary

Numeric string like "1234567" were marshalled to their float64 representation when creating/updating translation values, resulting in following error API request error 400 Expecting ``it`` translation to be a string.

This fix handles translation value to be in one of possible forms according to API specs: string or map[string]interface{}

[andrius-spotos]

@andrewLab-lokalise

[andrius-spotos]

@Dosexe