Fix binding of untagged struct fields

[flimzy]

This fixes a corner case with binding to structs with untagged fields. For example, given a struct type FooStruct, which implements the BindUnmarshaler interface, the following code did not work previously when passed to c.Bind():

type Fields struct {
    Foo FooStruct
}

But this would have:

type Fields struct {
    Foo FooStruct `query:"Foo"`
}

This PR fixes the former case to also work.

[coveralls]

Coverage decreased (-0.2%) to 86.149% when pulling 4146e43 on flimzy:untagged-structs into 63262b2 on labstack:master.

[vishr]

@flimzy You have some print statements in the code.

[flimzy]

Oops. I'll fix that and update the PR tomorrow. Thanks for checking.

[flimzy]

@vishr Should now be cleaned up.

[vishr]

@flimzy are you sure?

[flimzy]

:/

I forgot the -f flag with my push. Now it's updated.

[coveralls]

Coverage decreased (-0.1%) to 85.374% when pulling f98e483 on flimzy:untagged-structs into 9797cf4 on labstack:master.