Fix Time.xmlschema handling leading and trailing whitespaces

[byroot]

Followup: #66

This reduce the gain to only ~3.8x instead of ~4x, but still worth it.

2026-07-20 08:37:09.66407 +0200
2026-07-20 08:37:09.66407 +0200
ruby 4.0.5 (2026-05-20 revision 64336ffd0e) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
           xmlschema    68.139k i/100ms
            Time.new   510.172k i/100ms
                 opt   239.558k i/100ms
Calculating -------------------------------------
           xmlschema    667.174k (± 1.4%) i/s    (1.50 μs/i) -      3.339M in   5.004405s
            Time.new      5.539M (± 1.3%) i/s  (180.53 ns/i) -     28.059M in   5.065670s
                 opt      2.538M (± 0.7%) i/s  (394.01 ns/i) -     12.697M in   5.002588s

Comparison:
xmlschema:   667174.4 i/s
 Time.new:  5539140.9 i/s - 8.30x  faster
      opt:  2538001.1 i/s - 3.80x  faster