Make String#each_line work correctly with paragraph separator and chomp

Previously, it was including one newline when chomp was used,
which is inconsistent with IO#each_line behavior. This makes
behavior consistent with IO#each_line, chomping all paragraph
separators (multiple consecutive newlines), but not single
newlines.

Partially Fixes [Bug #18768]

string.c

@@ -8967,14 +8967,18 @@ rb_str_enumerate_lines(int argc, VALUE *argv, VALUE str, VALUE ary)
 	const char *eol = NULL;
 	subend = subptr;
 	while (subend < pend) {
+            long chomp_rslen = 0;
 	    do {
 		if (rb_enc_ascget(subend, pend, &n, enc) != '\r')
 		    n = 0;
 		rslen = n + rb_enc_mbclen(subend + n, pend, enc);
 		if (rb_enc_is_newline(subend + n, pend, enc)) {
 		    if (eol == subend) break;
 		    subend += rslen;
-		    if (subptr) eol = subend;
+                    if (subptr) {
+                        eol = subend;
+                        chomp_rslen = -rslen;
+                    }
 		}
 		else {
 		    if (!subptr) subptr = subend;
@@ -8983,8 +8987,9 @@ rb_str_enumerate_lines(int argc, VALUE *argv, VALUE str, VALUE ary)
 		rslen = 0;
 	    } while (subend < pend);
 	    if (!subptr) break;
+            if (rslen == 0) chomp_rslen = 0;
 	    line = rb_str_subseq(str, subptr - ptr,
-				 subend - subptr + (chomp ? 0 : rslen));
+                                 subend - subptr + (chomp ? chomp_rslen : rslen));
 	    if (ENUM_ELEM(ary, line)) {
 		str_mod_check(str, ptr, len);
 	    }

test/ruby/test_string.rb

@@ -1121,14 +1121,19 @@ def test_each_line_chomp
     assert_equal(S("world"), res[1])
 
     res = []
-    S("hello\n\n\nworld").each_line(S(''), chomp: true) {|x| res << x}
-    assert_equal(S("hello\n"), res[0])
-    assert_equal(S("world"),   res[1])
+    S("hello\n\n\nworld\n").each_line(S(''), chomp: true) {|x| res << x}
+    assert_equal(S("hello"), res[0])
+    assert_equal(S("world\n"), res[1])
 
     res = []
-    S("hello\r\n\r\nworld").each_line(S(''), chomp: true) {|x| res << x}
-    assert_equal(S("hello\r\n"), res[0])
-    assert_equal(S("world"),     res[1])
+    S("hello\r\n\r\nworld\r\n").each_line(S(''), chomp: true) {|x| res << x}
+    assert_equal(S("hello"), res[0])
+    assert_equal(S("world\r\n"), res[1])
+
+    res = []
+    S("hello\r\n\n\nworld").each_line(S(''), chomp: true) {|x| res << x}
+    assert_equal(S("hello"), res[0])
+    assert_equal(S("world"), res[1])
 
     res = []
     S("hello!world").each_line(S('!'), chomp: true) {|x| res << x}