[TEST] Make sure that match assertion throws error if run against an object

We had a REST test that relied on matching a json response against a regex. It worked but the match wasn't done against the actual json object, but its java map representation converted into a string by calling `toString`. Since all other clients test runners don't work in this case, as they try to match a json object against a regex, we should do the same and prevent it from working.

src/test/java/org/elasticsearch/test/rest/section/MatchAssertion.java

@@ -25,6 +25,7 @@ import java.util.regex.Pattern;
 
 import static org.elasticsearch.test.hamcrest.RegexMatcher.matches;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.notNullValue;
 import static org.junit.Assert.assertThat;
 
@@ -49,10 +50,12 @@ public class MatchAssertion extends Assertion {
         if (expectedValue instanceof String) {
             String expValue = ((String) expectedValue).trim();
             if (expValue.length() > 2 && expValue.startsWith("/") && expValue.endsWith("/")) {
+                assertThat("field [" + getField() + "] was expected to be of type String but is an instanceof [" + actualValue.getClass() + "]", actualValue, instanceOf(String.class));
+                String stringValue = (String) actualValue;
                 String regex = expValue.substring(1, expValue.length() - 1);
-                logger.trace("assert that [{}] matches [{}]", actualValue, regex);
+                logger.trace("assert that [{}] matches [{}]", stringValue, regex);
                 assertThat("field [" + getField() + "] was expected to match the provided regex but didn't",
-                        actualValue.toString(), matches(regex, Pattern.COMMENTS));
+                        stringValue, matches(regex, Pattern.COMMENTS));
                 return;
             }
         }