patch 8.0.0167: str2nr()/str2float() fail with negative values

Problem:    str2nr() and str2float() do not always work with negative values.
Solution:   Be more flexible about handling signs. (LemonBoy, closes #1332)
            Add more tests.
diff --git a/src/testdir/test_float_func.vim b/src/testdir/test_float_func.vim
index e51b83e..981d821 100644
--- a/src/testdir/test_float_func.vim
+++ b/src/testdir/test_float_func.vim
@@ -165,9 +165,22 @@
 
 func Test_str2float()
   call assert_equal('1.0', string(str2float('1')))
+  call assert_equal('1.0', string(str2float(' 1 ')))
+  call assert_equal('1.0', string(str2float(' 1.0 ')))
   call assert_equal('1.23', string(str2float('1.23')))
   call assert_equal('1.23', string(str2float('1.23abc')))
   call assert_equal('1.0e40', string(str2float('1e40')))
+
+  call assert_equal('1.0', string(str2float('+1')))
+  call assert_equal('1.0', string(str2float('+1')))
+  call assert_equal('1.0', string(str2float(' +1 ')))
+  call assert_equal('1.0', string(str2float(' + 1 ')))
+
+  call assert_equal('-1.0', string(str2float('-1')))
+  call assert_equal('-1.0', string(str2float('-1')))
+  call assert_equal('-1.0', string(str2float(' -1 ')))
+  call assert_equal('-1.0', string(str2float(' - 1 ')))
+
   call assert_equal('inf', string(str2float('1e1000')))
   call assert_equal('inf', string(str2float('inf')))
   call assert_equal('-inf', string(str2float('-inf')))