Ensure strict mode is used for Oj parser.

[edevil]

If we don't use strict mode strings that start with ":" are converted to symbols:

irb(main):002:0> Oj.load('{"log":"::1 - - [04/Aug/2016:16:15:13 +0000] \"OPTIONS * HTTP/1.0\" 200 126 \"-\" \"Apache/2.4.10 (Debian) PHP/5.6.24 (internal dummy connection)\"\n","stream":"stdout","time":"2016-08-04T16:15:13.726932176Z"}')
=> {"log"=>:":1 - - [04/Aug/2016:16:15:13 +0000] \"OPTIONS * HTTP/1.0\" 200 126 \"-\" \"Apache/2.4.10 (Debian) PHP/5.6.24 (internal dummy connection)\"\n", "stream"=>"stdout", "time"=>"2016-08-04T16:15:13.726932176Z"}

Using strict mode this does not happen:

irb(main):003:0> Oj.default_options = {bigdecimal_load: :float, :mode => :strict}
=> {:bigdecimal_load=>:float, :mode=>:strict}
irb(main):004:0> Oj.load('{"log":"::1 - - [04/Aug/2016:16:15:13 +0000] \"OPTIONS * HTTP/1.0\" 200 126 \"-\" \"Apache/2.4.10 (Debian) PHP/5.6.24 (internal dummy connection)\"\n","stream":"stdout","time":"2016-08-04T16:15:13.726932176Z"}')
=> {"log"=>"::1 - - [04/Aug/2016:16:15:13 +0000] \"OPTIONS * HTTP/1.0\" 200 126 \"-\" \"Apache/2.4.10 (Debian) PHP/5.6.24 (internal dummy connection)\"\n", "stream"=>"stdout", "time"=>"2016-08-04T16:15:13.726932176Z"}

Obtaining symbols instead of strings causes havoc for some downstream plugins.

[repeatedly]

Wow, I first knew this behaviour.
Good catch!

[repeatedly]

Tests are passed so exisiting json parsing works.

[tagomoris]

I found that this change changes the process global default behavior, including behavior of formatter_json.
Now formatter_json raises exception when it dumps values with symbols, with message "Failed to dump Symbol Object to JSON in strict mode.".

[tagomoris]

I re-checked the modes of Oj, and found that:

• Oj's default is object, to process preceeding ':' characters (and does much more others): http://www.ohler.com/dev/oj_misc/encoding_format.html
• any modes wasn't set for default_options in parser_json
• compat mode was set for default_options in formatter_json

And, Oj only has process global default_options. Boo.
I think we must use compat mode, or remove Oj from entire Fluentd.

[tagomoris]

I found this problem at here: https://gist.github.com/dimaqq/d65aea5d7e4f3fcb54fcc55747cf363e#file-gistfile1-txt-L159
from: #1222 (comment)