`JacksonXmlAnnotationIntrospector.findNamespace()` should properly merge namespace information

[cowtowncoder]

(note: offshoot of #628)

Looks like method findNamepace() in JacksonXmlAnnotationIntrospector is considering both @JsonProperty and @JacksonXmlNamespace (latter having precedence) but does not support case like so:

       @JsonProperty(value="value", namespace="uri:ns1")
        @JacksonXmlProperty(isAttribute=true)
        public int valueDefault = 42;

in which @JacksonXmlProperty does not define namespace (that is, is left as "", empty String).
In such case it should then return namespace value of @JsonProperty instead.

Ideally in future we could simply use methods from AnnotationIntrospection -- findNameForSerialization() and findNameForDeserializaiton -- which also expose "namespace", but on short term let's handle merging better.