=htp make coding specs more robust

On my machine I got:
GzipSpec: "invalid code lengths set" instead of  "incomplete dynamic bit length tree"
DeflateSpec: "unknown compression method" instead of "incorrect header check"

akka-http-tests/src/test/scala/akka/http/coding/CoderSpec.scala

@@ -28,7 +28,7 @@ abstract class CoderSpec extends WordSpec with CodecSpecSupport with Inspectors
   protected def newEncodedOutputStream(underlying: OutputStream): OutputStream
 
   case object AllDataAllowed extends Exception with NoStackTrace
-  protected def corruptInputMessage: Option[String]
+  protected def corruptInputCheck: Boolean = true
 
   def extraTests(): Unit = {}
 
@@ -55,10 +55,11 @@ abstract class CoderSpec extends WordSpec with CodecSpecSupport with Inspectors
       val request = HttpRequest(POST, entity = HttpEntity(largeText))
       Coder.decode(Coder.encode(request)) should equal(request)
     }
-    "throw an error on corrupt input" in {
-      corruptInputMessage foreach { message ⇒
-        val ex = the[DataFormatException] thrownBy ourDecode(corruptContent)
-        ex.getMessage should equal(message)
+    if (corruptInputCheck) {
+      "throw an error on corrupt input" in {
+        a[DataFormatException] should be thrownBy {
+          ourDecode(corruptContent)
+        }
       }
     }
     "not throw an error if a subsequent block is corrupt" in {

akka-http-tests/src/test/scala/akka/http/coding/DeflateSpec.scala

@@ -18,12 +18,11 @@ class DeflateSpec extends CoderSpec {
   protected def newEncodedOutputStream(underlying: OutputStream): OutputStream =
     new DeflaterOutputStream(underlying)
 
-  protected def corruptInputMessage: Option[String] = Some("invalid code -- missing end-of-block")
-
   override def extraTests(): Unit = {
     "throw early if header is corrupt" in {
-      val ex = the[DataFormatException] thrownBy ourDecode(ByteString(0, 1, 2, 3, 4))
-      ex.getMessage should equal("incorrect header check")
+      a[DataFormatException] should be thrownBy {
+        ourDecode(ByteString(0, 1, 2, 3, 4))
+      }
     }
   }
 }

akka-http-tests/src/test/scala/akka/http/coding/GzipSpec.scala

@@ -20,8 +20,6 @@ class GzipSpec extends CoderSpec {
   protected def newEncodedOutputStream(underlying: OutputStream): OutputStream =
     new GZIPOutputStream(underlying)
 
-  protected def corruptInputMessage: Option[String] = Some("invalid code lengths set")
-
   override def extraTests(): Unit = {
     "decode concatenated compressions" in {
       ourDecode(Seq(encode("Hello, "), encode("dear "), encode("User!")).join) should readAs("Hello, dear User!")

akka-http-tests/src/test/scala/akka/http/coding/NoCodingSpec.scala

@@ -9,7 +9,7 @@ import java.io.{ OutputStream, InputStream }
 class NoCodingSpec extends CoderSpec {
   protected def Coder: Coder with StreamDecoder = NoCoding
 
-  protected def corruptInputMessage: Option[String] = None // all input data is valid
+  override protected def corruptInputCheck = false
 
   protected def newEncodedOutputStream(underlying: OutputStream): OutputStream = underlying
   protected def newDecodedInputStream(underlying: InputStream): InputStream = underlying