Markdown writer: omit initial newlines in gfm `math` blocks.

When the input is of the form
```
$$
x=y
$$
```
we would previously get:
````
``` math

x=y
```
````
which is ugly.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/Text/Pandoc/Writers/Markdown/Inline.hs b/src/Text/Pandoc/Writers/Markdown/Inline.hs
index 98ac91cba..05106b259 100644
--- a/src/Text/Pandoc/Writers/Markdown/Inline.hs
+++ b/src/Text/Pandoc/Writers/Markdown/Inline.hs
@@ -537,7 +537,7 @@ inlineToMarkdown opts (Math DisplayMath str) = do
                         (url <> urlEncode str', str'))
           _ | isEnabled Ext_tex_math_gfm opts ->
                 return $ cr <> (literal "``` math"
-                             $$ literal str
+                             $$ literal (T.dropAround (=='\n') str)
                              $$ literal "```") <> cr
             | isEnabled Ext_tex_math_dollars opts ->
                 return $ delimited "$$" "$$" (literal str)