fixed type_convert. (Hash to Hash, Array to Array)

Both array class and hash class have "to_a" method.
So, 'value' is converted to array object when it is hash object.

* Norikra v1.0.1

```bash
# norikra-client query add ex "SELECT percentiles(num_field,{50,95}) FROM foo"
# echo '{ "num_field":1 }' | norikra-client event send foo
# norikra-client event fetch ex
{"time":"2014/06/01 20:24:10","percentiles(num_field)":{"95":1.0,"50":1.0}}
```
and in WebUI, the event shows
```
[1401623886,{"percentiles(num_field)":{"50":1,"95":1}}]
```

* Norikra v1.0.2,

```bash
# norikra start -d --logdir=/var/log/norikra  --pidfile=/var/run/norikra.pid
# norikra-client query add ex "SELECT percentiles(num_field,{50,95}) FROM foo"
# echo '{ "num_field":1 }' | norikra-client event send foo
# norikra-client event fetch ex
TypeError: can't convert Array into Hash
           merge at org/jruby/RubyHash.java:1763
           fetch at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/norikra-client-jruby-1.0.0-java/lib/norikra/client/cli.rb:168
            each at org/jruby/RubyArray.java:1613
           fetch at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/norikra-client-jruby-1.0.0-java/lib/norikra/client/cli.rb:167
            wrap at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/norikra-client-jruby-1.0.0-java/lib/norikra/client/cli.rb:14
           fetch at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/norikra-client-jruby-1.0.0-java/lib/norikra/client/cli.rb:166
             run at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor/command.rb:27
  invoke_command at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor/invocation.rb:126
        dispatch at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor.rb:359
          invoke at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor/invocation.rb:115
           event at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor.rb:235
             run at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor/command.rb:27
  invoke_command at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor/invocation.rb:126
        dispatch at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor.rb:359
           start at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/thor-0.19.1/lib/thor/base.rb:440
          (root) at /.rbenv/versions/jruby-1.7.11/lib/ruby/gems/shared/gems/norikra-client-jruby-1.0.0-java/bin/norikra-client:8
            load at org/jruby/RubyKernel.java:1101
          (root) at //.rbenv/versions/jruby-1.7.11/bin/norikra-client:23
```
And in WebUI, the result event shows
```
[1401623385,[["percentiles(num_field)",[["95",1],["50",1]]]]]
```

This pull request is to fix this problem.