[util-security] throw MatchError when case match is non-exhaustive

Problem

Supporting latest https://github.com/scala/scala/releases/tag/v2.12.13 includes
a feature change in the scala compiler that will fail to compile when case
matching is not exhaustive on tuples.

Issues
- scala/bug#10680
- scala/bug#10373
- scala/bug#10019

Resolved
- scala/scala#9163
- scala/scala#9147

Solution
Add `case _ => throw new MatchError` to case matching statement that is
non-exhaustive

JIRA Issues: SCALA-25

Differential Revision: https://phabricator.twitter.biz/D609046

util-security/src/main/scala/com/twitter/util/security/Credentials.scala

@@ -37,7 +37,10 @@ object Credentials {
     private[this] val key = "[\\w-]+".r
     private[this] val value = ".+".r
 
-    def auth: Parser[Tuple2[String, String]] = key ~ ":" ~ value ^^ { case k ~ ":" ~ v => (k, v) }
+    def auth: Parser[Tuple2[String, String]] = key ~ ":" ~ value ^^ {
+      case k ~ ":" ~ v => (k, v)
+      case _ => throw new MatchError
+    }
     def content: Parser[Map[String, String]] = rep(auth) ^^ { auths => Map(auths: _*) }
 
     def apply(in: String): Map[String, String] = {