ensure propper handling of multiple Content-Length headers (#4705)

When multiple Content-Length headers are received, error out if all received values do not match.
Return a single Content-Length header value.

spec/std/http/request_spec.cr

@@ -347,5 +347,33 @@ module HTTP
         request.host_with_port.should eq("host.example.org:3000")
       end
     end
+
+    it "doesn't raise on request with multiple Content_length headers" do
+      io = IO::Memory.new <<-REQ
+        GET / HTTP/1.1
+        Host: host
+        Content-Length: 5
+        Content-Length: 5
+        Content-Type: text/plain
+
+        abcde
+        REQ
+      HTTP::Request.from_io(io)
+    end
+
+    it "raises if request has multiple and differing content-length headers" do
+      io = IO::Memory.new <<-REQ
+        GET / HTTP/1.1
+        Host: host
+        Content-Length: 5
+        Content-Length: 6
+        Content-Type: text/plain
+
+        abcde
+        REQ
+      expect_raises(ArgumentError) do
+        HTTP::Request.from_io(io)
+      end
+    end
   end
 end

src/http/common.cr

@@ -26,8 +26,7 @@ module HTTP
         body = nil
         if body_type.prohibited?
           body = nil
-        elsif content_length = headers["Content-Length"]?
-          content_length = content_length.to_u64
+        elsif content_length = content_length(headers)
           if content_length != 0
             # Don't create IO for Content-Length == 0
             body = FixedLengthContent.new(io, content_length)
@@ -166,7 +165,13 @@ module HTTP
 
   # :nodoc:
   def self.content_length(headers)
-    headers["Content-Length"]?.try &.to_u64?
+    length_headers = headers.get? "Content-Length"
+    return nil unless length_headers
+    first_header = length_headers[0]
+    if length_headers.size > 1 && length_headers.any? { |header| header != first_header }
+      raise ArgumentError.new("Multiple Content-Length headers received did not match: #{length_headers}")
+    end
+    first_header.to_u64
   end
 
   # :nodoc: