v3: Use map[string] for maps when merged keys are also strings

[saracen]

The map type is map[string] when all keys in a map are strings. This adds an additional check to ensure that map[string] is also the map type used for merges, providing the map to be merged also only contains string keys.

This was reported by @nervo here: #139 (comment)

For example:

foo:
    &foo
    bar: baz

bar:
    <<: *foo
    baz: qux

At the moment returns a map[interface{}]interface{}, when ideally, it should be a map[string]interface{} because both bar and foo contain only string keys. This PR fixes this.

[saracen]

@niemeyer are you happy with this approach?

TestMerge is simplified, because all of the keys are strings. I've added additional tests for when they're not.