Add String(char *, unsigned) constructor

This constructor allows converting a buffer containing a
non-nul-terminated string to a String object, by explicitely passing the
length.

Author: matthijskooijman

Diff for: api/String.cpp

@@ -45,6 +45,12 @@ String::String(const char *cstr)
 	if (cstr) copy(cstr, strlen(cstr));
 }
 
+String::String(const char *cstr, unsigned int length)
+{
+	init();
+	if (cstr) copy(cstr, length);
+}
+
 String::String(const String &value)
 {
 	init();

Diff for: api/String.h

@@ -68,6 +68,7 @@ class String
 	// fails, the string will be marked as invalid (i.e. "if (s)" will
 	// be false).
 	String(const char *cstr = "");
+	String(const char *cstr, unsigned int length);
 	String(const String &str);
 	String(const __FlashStringHelper *str);
 	#if __cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__)