Faster time_value.rb

The multiplication of the value takes a long time when we can instead mutate and use the string value directly.

The `microsec` perf increases speed by 27% in the ideal case (which is the most common).

```
original_string = ".443959"

require 'benchmark/ips'

Benchmark.ips do |x|
  x.report("multiply") { 
    string = original_string.dup
    (string.to_r * 1_000_000).to_i 
  }
  x.report("new     ") { 
    string = original_string.dup
    if string && string.start_with?(".".freeze) && string.length == 7
      string[0] = ''.freeze
      string.to_i
    end
  }
  x.compare!
end

# Warming up --------------------------------------
#             multiply   125.783k i/100ms
#             new        146.543k i/100ms
# Calculating -------------------------------------
#             multiply      1.751M (± 3.3%) i/s -      8.805M in   5.033779s
#             new           2.225M (± 2.1%) i/s -     11.137M in   5.007110s

# Comparison:
#             new     :  2225289.7 i/s
#             multiply:  1751254.2 i/s - 1.27x  slower
```

activemodel/lib/active_model/type/helpers/time_value.rb

@@ -70,7 +70,13 @@ def new_time(year, mon, mday, hour, min, sec, microsec, offset = nil)
           # Doesn't handle time zones.
           def fast_string_to_time(string)
             if string =~ ISO_DATETIME
-              microsec = ($7.to_r * 1_000_000).to_i
+              microsec_part = $7
+              if microsec_part && microsec_part.start_with?(".") && microsec_part.length == 7
+                microsec_part[0] = ""
+                microsec = microsec_part.to_i
+              else
+                microsec = (microsec_part.to_r * 1_000_000).to_i
+              end
               new_time $1.to_i, $2.to_i, $3.to_i, $4.to_i, $5.to_i, $6.to_i, microsec
             end
           end