Don't print non-utf8 bodies

A lot of our services take in multipart data or binary data in the HTTP
body and when a wiremock predicate fails megabytes of binary data get
printed out to the console. Here I address this by not printing out a
string if it isn't valid utf8 and instead printing out the body length.

Another alternative may be to put in an upper limit to how large a body
will be printed and if it exceeds that size maybe saving the request to
a file for later analysis. But this change was so simple I figured I'd
open the PR first to start the discussion.

src/request.rs

@@ -44,7 +44,11 @@ impl fmt::Display for Request {
             let values = values.join(",");
             writeln!(f, "{}: {}", name, values)?;
         }
-        writeln!(f, "{}", String::from_utf8_lossy(&self.body))
+        if let Ok(body) = String::from_utf8(self.body.clone()) {
+            writeln!(f, "{}", body)
+        } else {
+            writeln!(f, "Body size is {} bytes", &self.body.len())
+        }
     }
 }