fix form-url-encoding data (body) for nested objects

[filipkowicz]

Fixes #149

Fixes #149 with calling urlencoding method recurrently on nested objects (preserving parent's keys)

Test provided for presented case.

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[filipkowicz]

I signed it!

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CLAs look good, thanks!

[filipkowicz]

hi, it makes this element usable for me. Please take a look