SoyMsgExtractor crashes when given a plural

Repro steps:
1) Create a soy template with a plural
```
/**
* Say hello to a list of people.
* @param names List of names of the people to say hello to.
*/
{template .helloNames}
{msg desc="names"}
{plural length($names)}
{case 0}Hello nobody!
{case 1}Hello {$names[0]}
{default}Hello sheeple
{/plural}
{/msg}
{/template}
```

2) `java -jar target/soy-2016-07-21-SoyMsgExtractor.jar --srcs example.soy --outputFile foo.xlf`

Expected result:
should output a valid xlf

Actual result:
```
java.lang.ClassCastException: com.google.template.soy.msgs.restricted.SoyMsgPluralPart cannot be cast to com.google.template.soy.msgs.restricted.SoyMsgPlaceholderPart
at com.google.template.soy.xliffmsgplugin.XliffGenerator.generateXliff(XliffGenerator.java:114)
at com.google.template.soy.xliffmsgplugin.XliffMsgPlugin.generateExtractedMsgsFile(XliffMsgPlugin.java:45)
at com.google.template.soy.msgs.SoyMsgBundleHandler.writeToExtractedMsgsFile(SoyMsgBundleHandler.java:182)
```

Reproduce the crash with the provided Soy template and SoyMsgExtractor command. Start at XliffGenerator.java:114, then inspect XliffMsgPlugin.java:45 and SoyMsgBundleHandler.java:182 to trace XLIFF generation. Done means the command completes without ClassCastException and writes a valid foo.xlf file.

Written by the indexing model from the issue text.