Allow bind with a map[string]string

[ItalyPaleAle]

It's always been possible to bind using an object of type map[string]string. For example:

obj := make(map[string]string)
c.Bind(&obj)

If the request body was formatted as JSON (or YAML), this worked fine. You'd get a dictionary with key-values. For example, with input:

{
    "foo": "bar",
    "hello": "world"
}

You'd get in the object:

• obj["foo"] = "bar"
• obj["hello"] = "world"

However, this did not work if the request's body was form-encoded (application/x-www-form-urlencoded or multipart/form-data).

This PR adds support for passing a pointer to map[string]string to Bind that works with: Forms (POST Forms, Multipart forms), JSON, YAML. It also works with query strings. (However, it does NOT work with XML, because the XML parser doesn't allow that).

Why is this useful? Sometimes, there are situations in which I'm expecting a key-value dictionary, but I do not know what keys I'll receive.

This PR includes tests.

[codecov]

Codecov Report

Merging #2484 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2484      +/-   ##
==========================================
+ Coverage   98.46%   98.48%   +0.01%     
==========================================
  Files          41       41              
  Lines        1952     1974      +22     
==========================================
+ Hits         1922     1944      +22     
  Misses         17       17              
  Partials       13       13              

Impacted Files	Coverage Δ
binding/form_mapping.go	100.00% <100.00%> (ø)

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[ItalyPaleAle]

Hi is there any update on this? @thinkerou or someone else?

[ItalyPaleAle]

Ping on this 😄 @thinkerou

[thinkerou]

lgtm