Introduce InvalidTrailerError.

lib/protocol/http/error.rb

@@ -26,5 +26,18 @@ def initialize(key)
 			# @attribute [String] key The header key that was duplicated.
 			attr :key
 		end
+
+		# Raised when an invalid trailer header is encountered in headers.
+		class InvalidTrailerError < Error
+			include BadRequest
+
+			# @parameter key [String] The trailer key that is invalid.
+			def initialize(key)
+				super("Invalid trailer key: #{key.inspect}")
+			end
+
+			# @attribute [String] key The trailer key that is invalid.
+			attr :key
+		end
 	end
 end

lib/protocol/http/headers.rb

@@ -407,7 +407,7 @@ def delete(key)
 				if policy = @policy[key]
 					# Check if we're adding to trailers and this header is allowed:
 					if trailer && !policy.trailer?
-						return false
+						raise InvalidTrailerError, key
 					end
 
 					if current_value = hash[key]
@@ -418,7 +418,7 @@ def delete(key)
 				else
 					# By default, headers are not allowed in trailers:
 					if trailer
-						return false
+						raise InvalidTrailerError, key
 					end
 
 					if hash.key?(key)
@@ -431,6 +431,8 @@ def delete(key)
 
 			# Compute a hash table of headers, where the keys are normalized to lower case and the values are normalized according to the policy for that header.
 			#
+			# This will enforce policy rules, such as merging multiple headers into arrays, or raising errors for duplicate headers.
+			# 
 			# @returns [Hash] A hash table of `{key, value}` pairs.
 			def to_h
 				unless @indexed

test/protocol/http/headers.rb

@@ -343,7 +343,7 @@
 				it "can't add a #{key.inspect} header in the trailer", unique: key do
 					trailer = headers.trailer!
 					headers.add(key, "example")
-					expect(headers).not.to be(:include?, key)
+					expect{headers.to_h}.to raise_exception(Protocol::HTTP::InvalidTrailerError)
 				end
 			end
 		end