http4s generator does not handle non-string headers

[blast-hardcheese]

Related to #150

Currently, server headers are handled as:

req.headers.get("x-header".ci).map(_.value).map(Json.fromString(_).as[Boolean])

... resulting in attempting to parse the JSON literal "false" as a Boolean, which can never be true. This is related to a similar issue in the akka-http generator, where X-Header: "" will be silently converted to an empty string, as circe Decoders are used similar to parser.parse(str).orElse(Json.fromString(str)).flatMap(_.as[Boolean]) to attempt to decode all possible x-scala-type overrides.

I believe the most effective way to disambiguate this usecase is to alter headersToHttp4s (and likely formToHttp4s, and any other place Json.fromString is used) by adding an additional parameter to each case in directivesFromParams, baseType: Type.Name, to explicitly handle String decoders vs other JSON literal values.

Test swagger spec:

swagger: '2.0'
paths:
  /test:
    post:
      operationId: createFoo
      parameters:
        - in: header
          name: x-header
          type: boolean
          required: true
        - in: header
          name: x-optional-header
          type: boolean
          required: false
      responses:
        200:
          description: stub