Return ErrResponseTooLarge instead of truncating response body

WithMaxResponseBody wrapped the body in io.LimitedReader, which returns EOF
at the cap, so Bytes/JSON/XML silently returned a truncated body with a nil
error despite the documented contract. Read one byte past the limit and
return the new ErrResponseTooLarge sentinel when exceeded; bodies exactly at
the limit still succeed.

client_options.go

@@ -89,8 +89,8 @@ func WithBalancer(opts ...balancer.Option) Option {
 
 // WithMaxResponseBody limits the number of bytes read from response bodies
 // by Response.Bytes (and by extension String, JSON, XML). If the response
-// body exceeds n bytes, reading stops and returns an error.
-// A value of 0 means no limit (the default).
+// body exceeds n bytes, reading returns ErrResponseTooLarge instead of
+// silently truncating. A value of 0 means no limit (the default).
 func WithMaxResponseBody(n int64) Option {
 	return func(o *clientOptions) { o.maxResponseBody = n }
 }