Fix various trim functions (#271)

Trim doesn't handle the case where the whole string should be trimmed,
due to being composed of only trimmable chars.
For example the string "\n" when trimmed with chars " \t\n\r" should
yield the empty string "" and not return "\n".
Some test cases were fixed and some were added.

src/string.cpp

@@ -480,7 +480,7 @@ namespace bx
 			}
 		}
 
-		return _str;
+		return StringView(_str.getTerm(), _str.getTerm() );
 	}
 
 	StringView strLTrimSpace(const StringView& _str)
@@ -523,7 +523,9 @@ namespace bx
 				{
 					return StringView(ptr, ii + 1);
 				}
-			}
+		}
+
+			return StringView(_str.getPtr(), _str.getPtr());
 		}
 
 		return _str;
@@ -542,6 +544,8 @@ namespace bx
 					return StringView(ptr, ii + 1);
 				}
 			}
+			
+			return StringView(_str.getPtr(), _str.getPtr());
 		}
 
 		return _str;