Make BaseRequest.contentType use null rather than -1 as a flag value.

pkg/http/CHANGELOG.md

Index: pkg/http/CHANGELOG.md
diff --git a/pkg/http/CHANGELOG.md b/pkg/http/CHANGELOG.md
new file mode 100644
index 0000000000000000000000000000000000000000..0a9b4bac3a2260cf4bf5ed6e2391750e0eb3cfbd
--- /dev/null
+++ b/pkg/http/CHANGELOG.md
@@ -0,0 +1,9 @@
+## 0.10.0
+
+* Make `BaseRequest.contentLength` and `BaseResponse.contentLength` use `null`
+  to indicate an unknown content length rather than -1.
+
+* The `contentLength` parameter to `new BaseResponse` is now named rather than
+  positional.
+
+* Make request headers case-insensitive.