RFR: 8283544: HttpClient GET method adds Content-Length: 0 header

[Conor Cleary]

**Issue**
When using the `HttpClient.send()` to send a GET request created using the `HttpRequest.newBuilder()`, a `Content-length: 0` header is set. This behaviour causes issues with many services as a body related header is usually not expected to be included with a GET request. 

**Solution**
`Http1Request.java` was modified so that when the request method is a GET, a `Content-length` header is not added to the request. However, if a developer chooses to include a body in a GET request (though it is generally considered bad practice), a `Content-length` header with the appropriate value will be added.

-------------

Commit messages:
 - 8283544: HttpClient GET method adds Content-Length: 0 header

Changes: https://git.openjdk.java.net/jdk/pull/8017/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8017&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283544
  Stats: 168 lines in 2 files changed: 159 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8017.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8017/head:pull/8017

PR: https://git.openjdk.java.net/jdk/pull/8017