Changed, that the `<<` operator is used in Logger, instead of `+=`

Dramatically improves the performance of the code where strings are concatenated, by changing the "+=" for "<<".

Sample profiling of line 50 on logger.rb with a 20,000 lines json response from Elasticsearch:

    Before the change
    99.74%  48.04 seconds String#each_line

    After the change
    74.91%  0.11 seconds String#each_line

Closes #959