Fix for headers priority setting between clients, request and body values

[malcommac]

The following PR fixes the issue found in #62 with headers of a request.
Due to a design issue request.headers are removed automatically when a custom request.body is set.

This is an example of the issue:

try await HTTPRequest {
  $0.headers = HTTPHeaders([
    authorization:"Bearer \(token)",
    .contentType:"text/json"
  ])
  $0.body = try .multipart(boundary: nil, { form in
    try form.add(string: name, name: "name")
    try form.add(string: surname, name: "surname")
    try form.add(string: email, name: "email")
  }
})
}.fetch()

In this example, we set both the Authorization and Content-Type headers.
However, when we set the .body of the request to a multi-part form-encoded, the specific body configuration sets its additional headers (i.e., Content-Length) by replacing the entire HTTPHeaders object with its own instead of merging values.

Our fix decuple the request.headers and request.body.headers in two different objects.
When the URLRequest is generated from a client, values are merged in the following order:

• client's headers (customized by the user, shared among all requests executed in that client instance)
• request.body's headers (automatically set and optionally customized by the user)
• request.header's (set by the user)

A specific unit test was also added to validate the following scenario.