Contextualize deser with CreatorProperty, not intermediate prop
Merge pull request #470 from aarondav/context

Contextualize deser with CreatorProperty, not intermediate prop
Avoid setting explName to true if name is null in POJOPropertyBuilder

This change enables backwards compatibility of Jackson 2.4.0 with
modules relying on the 2.3 API (e.g., jackson-module-scala 2.3.X).

Prior to this change, old versions of jackson-module-scala would
receive this exception, when running with Jacskon 2.4:
```
java.lang.IllegalArgumentException: Can not pass true for 'explName' if name is null/empty
    at com.fasterxml.jackson.databind.introspect.POJOPropertyBuilder$Linked.<init>(POJOPropertyBuilder.java:955)
```

Note that this behavior is effectively what jackson-module-scala does in 2.4.0:
https://github.com/FasterXML/jackson-module-scala/blob/master/src/main/scala/com/fasterxml/jackson/module/scala/introspect/ScalaPropertiesCollector.scala#L211