feat(json): convert Blobs to array of byte values

Similiar to how Vim does it, but to be consistent with how Nvim encodes
lists, add a space after every comma.

runtime/doc/eval.txt

@@ -6002,6 +6002,7 @@ json_encode({expr})					*json_encode()*
 		surrogate pairs (such strings are not valid UTF-8 strings).
 		Non-printable characters are converted into "\u1234" escapes
 		or special escapes like "\t", other are dumped as-is.
+		|Blob|s are converted to arrays of the individual bytes.
 
 keys({dict})						*keys()*
 		Return a |List| with all the keys of {dict}.  The |List| is in

src/nvim/eval/encode.c

@@ -729,7 +729,25 @@ static inline int convert_to_json_string(garray_T *const gap,
 
 #undef TYPVAL_ENCODE_CONV_BLOB
 #define TYPVAL_ENCODE_CONV_BLOB(tv, blob, len) \
-    abort() /* TODO(seandewar) */ \
+    do { \
+      const blob_T *const blob_ = (blob); \
+      const int len_ = (len); \
+      if (len_ == 0) { \
+        ga_concat(gap, "[]"); \
+      } else { \
+        ga_append(gap, '['); \
+        char numbuf[NUMBUFLEN]; \
+        for (int i_ = 0; i_ < len_; i_++) { \
+          if (i_ > 0) { \
+            ga_concat(gap, ", "); \
+          } \
+          vim_snprintf((char *)numbuf, ARRAY_SIZE(numbuf), "%d", \
+                       (int)tv_blob_get(blob_, i_)); \
+          ga_concat(gap, numbuf); \
+        } \
+        ga_append(gap, ']'); \
+      } \
+    } while (0)
 
 #undef TYPVAL_ENCODE_CONV_FUNC_START
 #define TYPVAL_ENCODE_CONV_FUNC_START(tv, fun) \

test/functional/eval/json_functions_spec.lua

@@ -538,6 +538,11 @@ describe('json_encode() function', function()
     eq('"þÿþ"', funcs.json_encode('þÿþ'))
   end)
 
+  it('dumps blobs', function()
+    eq('[]', eval('json_encode(0z)'))
+    eq('[222, 173, 190, 239]', eval('json_encode(0zDEADBEEF)'))
+  end)
+
   it('dumps numbers', function()
     eq('0', funcs.json_encode(0))
     eq('10', funcs.json_encode(10))
@@ -769,6 +774,10 @@ describe('json_encode() function', function()
     eq('""', eval('json_encode($XXX_UNEXISTENT_VAR_XXX)'))
   end)
 
+  it('can dump NULL blob', function()
+    eq('[]', eval('json_encode(v:_null_blob)'))
+  end)
+
   it('can dump NULL list', function()
     eq('[]', eval('json_encode(v:_null_list)'))
   end)