[Rust] client codegen ignores all fields that are of type "formData"

[euank]

Description

I think the title for this one is fairly self-descriptive.

I intend to fix this one myself fairly soon.

[dbldippy]

Unfortunately, it seems that this issue persists with generator option "library": "reqwest" #1258
@bcourtine

[bcourtine]

@myk3v Thanks for the ping. I will look at this.