Align type mapping for PostGIS types across decoding plug-ins

From PR #1079:

During snapshot a record exists where it was inserted using the value (0,0),(1,1), the emitted value is a bytebuffer with the contents of (0.0,0.0),(1.0,1.0). I want to point out that the emitted value contains a .0 for each of the coordinates for the box. The odd behavior comes into play during streaming however, if using wal2json or pgoutput, the emitted value is a bytebuffer with the same contents of that from the snapshot; however, when using decoderbufs its actually a bytebuffer with the contents of (0,0),(1,1), notice the missing .0.

DecoderBufs should be adjusted so also the leading 0 digits are exposed and for consumers the representation is the same during snapshotting and streaming for all the three decoders.