Simplify String concatenation

[stsypanov]

As it follows from
http://alblue.bandlem.com/2016/04/jmh-stringbuffer-stringbuilder.html
and
https://habr.com/ru/post/330220/
chained StringBuilder::append is much faster than conventional sb.append(); sb.append().

Assyming that expression like String str = str1 + str2 + str3; at runtime will behave as chained StringBuilder::append we can simplify usage of StringBuilder in particular cases like the one of this PR.

Here we have 2 cases:

• if expression is false
• if expression is true

In the first case we don't need SB at all, as resulting expression produced by sb.toString() equals this.contentType.
And in the opposite case we have non-interrupted StringBuilder::append chain (later simplifiable to String concatenation with +) and thus more simple code and better performance.