Test against latest Framework 5.1 snapshot

This updates REST Docs to test against Framework's latest 5.1
snapshot. Due to a bug in the behaviour of HttpHeaders' key set [1],
HeaderRemovingOperationPreprocessor has been updated to no longer use
it when removing headers from a request or response prior to it being
documented.

[1] https://github.com/spring-projects/spring-framework/issues/22821

spring-restdocs-webtestclient/build.gradle

@@ -19,6 +19,6 @@ test {
 matrixTest {
 	springFramework {
 		group = 'org.springframework'
-		versions = ['5.1.0.BUILD-SNAPSHOT']
+		versions = ['5.1.10.BUILD-SNAPSHOT']
 	}
 }