Fail if the key path is empty. (#57)

We should fail on input like `=foo` as the key path is ambiguous.

lib/protocol/http/url.rb

@@ -65,9 +65,13 @@ def self.scan(string)
 			end
 
 			def self.split(name)
-				name.scan(/([^\[]+)|(?:\[(.*?)\])/).flatten!.compact!
+				name.scan(/([^\[]+)|(?:\[(.*?)\])/)&.tap do |parts|
+					parts.flatten!
+					parts.compact!
+				end
 			end
 
+			# Assign a value to a nested hash.
 			def self.assign(keys, value, parent)
 				top, *middle = keys
 
@@ -95,6 +99,10 @@ def self.decode(string, maximum = 8, symbolize_keys: false)
 				self.scan(string) do |name, value|
 					keys = self.split(name)
 
+					if keys.empty?
+						raise ArgumentError, "Invalid key path: #{name.inspect}!"
+					end
+
 					if keys.size > maximum
 						raise ArgumentError, "Key length exceeded limit!"
 					end

test/protocol/http/url.rb

@@ -70,6 +70,12 @@
 				Protocol::HTTP::URL.decode("a[b][c][d][e][f][g][h][i]=10")
 			end.to raise_exception(ArgumentError, message: be =~ /Key length exceeded/)
 		end
+
+		it "fails with missing key" do
+			expect do
+				Protocol::HTTP::URL.decode("=foo")
+			end.to raise_exception(ArgumentError, message: be =~ /Invalid key/)
+		end
 	end
 
 	with '.unescape' do