invalid boolean values in generated json for table entries

[cc10512]

From @vgurevich:
Attached is a program and the JSON, produced by p4c-bm2-ss that result in BMv2 termination with the following message:

$ sudo simple_switch -i 0@veth0 -i 1@veth2 -i 2@veth4 --log-console --dump-packet-data 64 l3_switch.json 
Thrift port was not specified, will use 9090
Calling target program-options parser
[16:31:18.454] [bmv2] [D] [thread 7495] Set default entry for table 'tbl_act': act - 
[16:31:18.454] [bmv2] [D] [thread 7495] Set default entry for table 'ipv4_lpm': drop - 
[16:31:18.455] [bmv2] [D] [thread 7495] Set default entry for table 'forward': drop - 
terminate called after throwing an instance of 'std::runtime_error'
  what():  Value is not convertible to bool.
Aborted (core dumped)

The problem is in the code that the compiler generates for the table initialization entries, e.g.

{
    "match_type" : "valid",
     "key" : "true"
},

The right way to generate it is to use booleans instead of strings. Removing the quotation marks around "true"s and "false"s fixes the problem.