Gson 2.8.9

• Make OSGi bundle's dependency on sun.misc optional (#1993).
• Deprecate Gson.excluder() exposing internal Excluder class (#1986).
• Prevent Java deserialization of internal classes (#1991).
• Improve number strategy implementation (#1987).
• Fix LongSerializationPolicy null handling being inconsistent with Gson (#1990).
• Support arbitrary Number implementation for Object and Number deserialization (#1290).
• Bump proguard-maven-plugin from 2.4.0 to 2.5.1 (#1980).
• Don't exclude static local classes (#1969).
• Fix RuntimeTypeAdapterFactory depending on internal Streams class (#1959).
• Improve Maven build (#1964).
• Make dependency on java.sql optional (#1707).