Added RFC2822 date format test cases

tests/date/rfc2822.c

+#include "clar_libgit2.h"
+
+#include "util.h"
+
+void test_date_rfc2822__format_rfc2822_no_offset(void)
+{
+	git_time t = {1397031663, 0};
+	char buf[GIT_DATE_RFC2822_SZ];
+
+	cl_git_pass(git__date_rfc2822_fmt(buf, sizeof(buf), &t));
+	cl_assert(strcmp(buf, "Wed, 9 Apr 2014 08:21:03 +0000") == 0);
+}
+
+void test_date_rfc2822__format_rfc2822_positive_offset(void)
+{
+	git_time t = {1397031663, 120};
+	char buf[GIT_DATE_RFC2822_SZ];
+
+	cl_git_pass(git__date_rfc2822_fmt(buf, sizeof(buf), &t));
+	cl_assert(strcmp(buf, "Wed, 9 Apr 2014 10:21:03 +0200") == 0);
+}
+
+void test_date_rfc2822__format_rfc2822_negative_offset(void)
+{
+	git_time t = {1397031663, -120};
+	char buf[GIT_DATE_RFC2822_SZ];
+
+	cl_git_pass(git__date_rfc2822_fmt(buf, sizeof(buf), &t));
+	cl_assert(strcmp(buf, "Wed, 9 Apr 2014 06:21:03 -0200") == 0);
+}
+
+void test_date_rfc2822__format_rfc2822_buffer_too_small(void)
+{
+	// "Wed, 10 Apr 2014 08:21:03 +0000"
+	git_time t = {1397031663 + 86400, 0};
+	char buf[GIT_DATE_RFC2822_SZ-1];
+
+	cl_git_fail(git__date_rfc2822_fmt(buf, sizeof(buf), &t));
+}
+