Tolerate changes to request body reading in Framework 5.1

Previously, MockHttpServletRequest allowed its body to be ready
multiple times using its InputStream. As of recent Spring Framework
5.1 snapshots, this is no longer the case.

This commit moves to directly accessing the request's content as a
byte array using getContentAsByteArray() which was introduced in
Framework 5.0.

Closes gh-515

spring-restdocs-webtestclient/build.gradle

@@ -14,4 +14,11 @@ dependencies {
 
 test {
 	jvmArgs "-javaagent:${configurations.jacoco.asPath}=destfile=${buildDir}/jacoco.exec,includes=org.springframework.restdocs.*"
+}
+
+matrixTest {
+	springFramework {
+		group = 'org.springframework'
+		versions = ['5.1.0.BUILD-SNAPSHOT']
+	}
 }