MultipartData needs $

[juancarlospaco]

Summary

• httpclient.MultipartData needs a $ to stringuify.
• Create a httpclient.`$`*(data: httpclient.MultipartData): string.
• MultipartData is just a seq[string] so it should not be too complex to add.

Actual error of doing echo on a MultipartData:

Error: type mismatch: got <MultipartData>
expression: `$`(multipart_data)

Its useful for debugging and learning.
🙂