`http::basic_fields::equal_range` should return `boost::range::iterator_range`

In terms of improving Beast's semantic meaning, using a [`std::pair`](https://www.boost.org/doc/libs/1_67_0/libs/beast/doc/html/beast/ref/boost__beast__http__basic_fields/equal_range.html) as the return type for `basic_fields::equal_range` can lead to a hard-to-use API.

It's suggested that Beast instead return what's in Boost.Range for better interop with existing Boost libs.