feat(headers): Added an X-Proxy-Cache header

src/main/scala/com/imageintelligence/http4c/headers/`X-Proxy-Cache`.scala

@@ -0,0 +1,39 @@
+package com.imageintelligence.http4c.headers
+
+import scalaz._, Scalaz._
+import org.http4s.util.Writer
+import org.http4s.dsl._
+import org.http4s._
+
+final case class `X-Proxy-Cache`(cacheHit: Boolean) extends Header.Parsed {
+  override def key = `X-Proxy-Cache`
+  override def renderValue(writer: Writer): writer.type = {
+    if (cacheHit) {
+      writer.append("HIT")
+    } else {
+      writer.append("MISS")
+    }
+  }
+}
+
+object `X-Proxy-Cache` extends HeaderKey.Singleton {
+  type HeaderT = `X-Proxy-Cache`
+  def name = "X-Proxy-Cache".ci
+
+  def parse(s: String): ParseResult[`X-Proxy-Cache`] = {
+    s match {
+      case "HIT" => `X-Proxy-Cache`(true).right
+      case "MISS" => `X-Proxy-Cache`(false).right
+      case other => {
+        val e = s"$other is not a valid X-Proxy-Cache header"
+        ParseFailure.apply(e, e).left
+      }
+    }
+  }
+
+  def matchHeader(header: Header): Option[`X-Proxy-Cache`] = {
+    if (header.name == name)
+      parse(header.value).toOption
+    else None
+  }
+}

src/test/scala/com/imageintelligence/http4c/examples/ExampleUserService.scala

@@ -1,6 +1,7 @@
 package com.imageintelligence.http4c.examples
 
 import com.imageintelligence.http4c.DslHelpers
+import com.imageintelligence.http4c.headers.`X-Proxy-Cache`
 import org.http4s.ParseFailure
 import org.http4s.HttpService
 import org.http4s.dsl._
@@ -37,7 +38,7 @@ object ExampleUserService {
   val service = HttpService {
     case req @ GET -> Root / UserIdMatcher(userId) :? GenderQueryParam(gender)  => {
       userId match {
-        case \/-(id) => Ok(id.value)
+        case \/-(id) => Ok(id.value).putHeaders(`X-Proxy-Cache`(false))
         case -\/(error) => BadRequest(error)
       }
     }