fix(dynamic-typing): accept an explicit leading plus sign on numbers

The FLOAT gate in testFloat allowed only a minus sign on the mantissa, while
the exponent right after it already allowed either sign. dynamicTyping
therefore left +42 and +1.5E-03 as strings and converted -42 and -1.5E-03,
silently producing columns that mix numbers and strings depending on how each
row wrote its sign.

parseFloat already accepts a leading plus and returns the right value, so the
regex was stricter than the conversion it guards. Allow either sign on the
mantissa, matching the exponent.

The 2^53 precision guard is unchanged, and a plus that is not a sign is still
rejected.

Fixes #1070

tests/test-cases.js

@@ -932,6 +932,24 @@ var PARSE_TESTS = [
 			errors: []
 		}
 	},
+	{
+		description: "Dynamic typing converts numeric literals with an explicit plus sign",
+		input: '+1,+2.2,+1e3\r\n+4,+4.5,+1.5E-03\r\n+.5,+9007199254740991,+9007199254740992',
+		config: { dynamicTyping: true },
+		expected: {
+			data: [[1, 2.2, 1000], [4, 4.5, 0.0015], [0.5, 9007199254740991, "+9007199254740992"]],
+			errors: []
+		}
+	},
+	{
+		description: "Dynamic typing doesn't convert a plus sign that isn't a sign",
+		input: '+,++1,+-1\r\n+a,1+2,+ 1',
+		config: { dynamicTyping: true },
+		expected: {
+			data: [["+", "++1", "+-1"], ["+a", "1+2", "+ 1"]],
+			errors: []
+		}
+	},
 	{
 		description: "Dynamic typing converts boolean literals",
 		input: 'true,false,T,F,TRUE,FALSE,True,False',