IOS-2543 Tighten the CWPart API

PantomimeFramework/PantomimeFramework/CWPart.h

@@ -374,7 +374,7 @@
 		      for the specified key,
   @param theKey  The key for which the value will be set.
 */
-- (void) setParameter: (id _Nonnull) theParameter  forKey: (NSString * _Nonnull) theKey;
+- (void) setParameter: (NSString * _Nullable) theParameter  forKey: (NSString * _Nonnull) theKey;
 
 /*!
   @method allHeaders

pantomime-lib/Framework/Pantomime/CWPart.m

@@ -710,7 +710,7 @@ static int currentPartVersion = 2;
   return [_parameters objectForKey: theKey];
 }
 
-- (void) setParameter: (id) theParameter  forKey: (NSString *) theKey
+- (void) setParameter: (NSString *) theParameter  forKey: (NSString *) theKey
 {
   if (theParameter)
     {