Fix Scala 2.12.0-M4 pattern match warnings

[ashawley]

After updating the build to 2.12.0-M4 for JDK 1.8 and higher in #100, I noticed some new pattern match warnings in the build:

[warn] src/test/scala/scala/xml/PatternMatching.scala:93: match may not be exhaustive.
[warn] It would fail on the following inputs: ((_ : Serializable), "bar"), ((_ : Serializable), _), (Node(), "foo"), (Node(), _), (Object(), _), (_, "bar"), (_, "foo"), (_, String()), (_, _)
[warn]     assertTrue((body: AnyRef, "foo") match {
[warn]                ^
[warn] src/test/scala/scala/xml/PatternMatching.scala:98: match may not be exhaustive.
[warn] It would fail on the following inputs: ((_ : scala.xml.Node with Serializable), _), (_, "foo"), (_, _)
[warn]     assertTrue((body, "foo") match {
[warn]                ^
[warn] src/test/scala/scala/xml/PatternMatching.scala:103: match may not be exhaustive.
[warn] It would fail on the following inputs: ((_ : Serializable), _), (Node(), _), (Object(), _), (_, "foo"), (_, _)
[warn]     assertTrue((body: AnyRef, "foo") match {
[warn]                ^
[warn] src/test/scala/scala/xml/PatternMatching.scala:108: match may not be exhaustive.
[warn] It would fail on the following inputs: ((_ : Serializable), _), (Node(), _), (Object(), _), (_, "foo"), (_, _)
[warn]     assertTrue((body: AnyRef, "foo") match {
[warn]                ^
[warn] four warnings found

I added some wildcard case matches to silence the warnings, and it doesn't appear they broke the tests nor undermined their purpose.

[SethTisue]

LGTM

[biswanaths]

Merged. Thank you @ashawley and @SethTisue .