Parse request parameters

[eugene-borovov]

@Sephster I have implemented my proposal.

• Params parsed to non-empty string
• Scalar params cast to string
• Empty string treats as null
• Non-scalar params throws invalidRequest
• Remove string checks
• Null checks have been shorten to null-coalesce operator

I will be glad if it helps.

[Sephster]

@eugene-borovov thank you very much for this PR. Saved me a great deal of time and looks good. Thank you