fixed two bugs with prototext format

fixed two bugs with prototext format

The first bug was in InitUnsharedWeightsNet. Proto var was of type
ostringstream, which converted the bool bias_term into an int. I
wrote an inline conditional to convert the term into a string.
This allows backwards compatibility with earlier prototext
versions (e.g. version 2.3.0 on Redhat was failing without this).

The second bug was in the syntax for repeated bool parameters,
assigned to the propagate_down parameter. The style used for e.g.
propagate_down: [true,true] does not work with earlier prototext
versions (failed with version 2.3.0 on Redhat). New syntax works
for all versions.