performance optimization & bugfix

[d-g-laptev]

Before:

Benchmark:

pkg: github.com/antchfx/xpath
Benchmark_NormalizeSpaceFunc-4    478104	     2441 ns/op	    305 B/op	      9 allocs/op
Benchmark_ConcatFunc-4           3489579	     346 ns/op	    194 B/op	      10 allocs/op
PASS

Test: (unicode space "\u00a0" wasn't processed correctly)

=== RUN   Test_NormalizeSpaceFunc
--- FAIL: Test_NormalizeSpaceFunc (0.00s)
    func_test.go:25: unexpected result 'loooooooonnnnnnngggggggg tes   t strin  g'
FAIL

After:

Benchmark:

pkg: github.com/antchfx/xpath
Benchmark_NormalizeSpaceFunc-4   2447773	      453 ns/op	    240 B/op	      4 allocs/op
Benchmark_ConcatFunc-4           4704387	      250 ns/op	    152 B/op	      8 allocs/op
PASS

Test:

=== RUN   Test_NormalizeSpaceFunc
--- PASS: Test_NormalizeSpaceFunc (0.00s)
PASS

[coveralls]

Coverage increased (+0.4%) to 76.112% when pulling 31b43ad on d-g-laptev:master into 51d8936 on antchfx:master.