Fix prefix/suffix/equal matcher to trim whitespaces

- Prefix matcher will trim leading whitespaces only when the pattern
  doesn't start with a whitespace
- Suffix matcher will trim trailing whitespaces only when the pattern
  doesn't end with a whitespace
- Equal matcher will trim leading whitespaces only when the pattern
  doesn't start with a whitespace, and trim trailing whitespaces only
  when the pattern doesn't end with a whitespace

Previously, only suffix matcher would trim whitespaces unconditionally.

Fix #1894

src/pattern_test.go

@@ -98,6 +98,9 @@ func TestEqual(t *testing.T) {
 	}
 	match("ABC", -1, -1)
 	match("AbC", 0, 3)
+	match("AbC  ", 0, 3)
+	match(" AbC ", 1, 4)
+	match("  AbC", 2, 5)
 }
 
 func TestCaseSensitivity(t *testing.T) {