Upgrade to Jackson 2.6.4

Changes, core
Databind

    #984: JsonStreamContexts are not build the same way for write.. and convert methods
    #989: Deserialization from "{}" to java.lang.Object causes "out of END_OBJECT token" error
    #1003: JsonTypeInfo.As.EXTERNAL_PROPERTY does not work with a Delegate
    #1004: Synthetic constructors confusing Jackson data binding
    #1013: @JsonUnwrapped is not treated as assuming @JsonProperty("")
    #1031: Problem with case-insensitive deserialization

Changes, dataformats
Avro

    #23: MapWriteContext.createChildObjectContext() should call _schema.getValueType()
    #26: Should indicate type that was requested when complaining about Any not being supported
    #27: Support serialization of BigDecimal values

CSV

    #90: Unexpected output with arrays starting with a null/empty element
    #98: Escape char is not being escaped during serialization

XML

    #171: @JacksonXmlRootElement malfunction in multi-thread environment
    #172: XML INDENT_OUTPUT property fails to add newline/indent initial elements

Changes, datatypes
JDK8

    #20: fails to serialize empty Optional with @JsonUnwrapped annotation
    #21: Manifest file missing OSGi import for com.fasterxml.jackson.databind.introspect
    #23: Fix an NPE for Optional<Boolean> when coercing from empty JSON String
    #25: OptionalDoubleSerializer does not override acceptJsonFormatVisitor
    #26: @JsonUnwrapped on Optional value does not propagate prefix to schema

Java 8 Date/Time

    #50: Instant schema representation is incorrect for timestamps serialization

Changes, other
Jackson jr

    #27: JSON.Feature.READ_JSON_ARRAYS_AS_JAVA_ARRAYS does not work