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] spring-projects/spring-framework#22821

spring-restdocs-core/build.gradle

@@ -67,6 +67,6 @@ test {
 matrixTest {
 	springFramework {
 		group = 'org.springframework'
-		versions = ['5.1.0.BUILD-SNAPSHOT']
+		versions = ['5.1.10.BUILD-SNAPSHOT']
 	}
 }

spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/preprocess/HeaderRemovingOperationPreprocessor.java

@@ -16,7 +16,8 @@
 
 package org.springframework.restdocs.operation.preprocess;
 
-import java.util.Iterator;
+import java.util.List;
+import java.util.Map.Entry;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.restdocs.operation.OperationRequest;
@@ -55,11 +56,9 @@ public OperationRequest preprocess(OperationRequest request) {
 
 	private HttpHeaders removeHeaders(HttpHeaders originalHeaders) {
 		HttpHeaders processedHeaders = new HttpHeaders();
-		processedHeaders.putAll(originalHeaders);
-		Iterator<String> headers = processedHeaders.keySet().iterator();
-		while (headers.hasNext()) {
-			if (this.headerFilter.excludeHeader(headers.next())) {
-				headers.remove();
+		for (Entry<String, List<String>> header : originalHeaders.entrySet()) {
+			if (!this.headerFilter.excludeHeader(header.getKey())) {
+				processedHeaders.put(header.getKey(), header.getValue());
 			}
 		}
 		return processedHeaders;

spring-restdocs-mockmvc/build.gradle

@@ -21,6 +21,6 @@ test {
 matrixTest {
 	springFramework {
 		group = 'org.springframework'
-		versions = ['5.1.0.BUILD-SNAPSHOT']
+		versions = ['5.1.10.BUILD-SNAPSHOT']
 	}
 }

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']
 	}
 }