[#36] i/o: fix bug in usage of std::isspace

Before, `std::isspace` was passed regular, possibly signed `char`s.
This is not supported by `std::isspace`, however, and causes incorrect
behaviour on some systems. As per cppreference:

>  The behavior [of `std::isspace`] is undefined if the value of ch
>  is not representable as unsigned char and is not equal to EOF.

-> see https://en.cppreference.com/w/cpp/string/byte/isspace

(cherry picked from commit 69cd380)

source/archive/archive_ascii.cc

@@ -47,7 +47,7 @@ namespace phoenix {
 		auto line = input.get_line();
 
 		// Compatibility fix for binary data in ASCII archives.
-		while (std::isspace(line[0])) {
+		while (std::isspace(static_cast<unsigned char>(line[0]))) {
 			line = line.substr(1);
 		}
 

source/buffer.cc

@@ -301,7 +301,7 @@ namespace phoenix {
 		(void) get_char(); // ignore the <LF> character itself
 
 		if (skip_whitespace) {
-			auto count = mismatch([](char chr) { return !std::isspace(chr); });
+			auto count = mismatch([](char chr) { return !std::isspace(static_cast<unsigned char>(chr)); });
 			if (count == -1) {
 				position(limit()); // the end of the buffer has been reached
 			} else {