Replace spaces in arguments with octal code 040

Modifying build_arg_string function to replace blank space
characters with octal code 040, and added a doctest to
check various combinations with single and double quotes
included.

pygmt/helpers/utils.py

@@ -107,7 +107,7 @@ def dummy_context(arg):
 
 
 def build_arg_string(kwargs):
- """
+ r"""
  Transform keyword arguments into a GMT argument string.
 
  Make sure all arguments have been previously converted to a string
@@ -159,6 +159,15 @@ def build_arg_string(kwargs):
  ... )
  ... )
  -BWSen -Bxaf -Byaf -I1/1p,blue -I2/0.25p,blue -JX4i -R1/2/3/4
+ >>> print(
+ ... build_arg_string(
+ ... dict(
+ ... B=["af", "WSne+tBlank Space"],
+ ... F='+t"Empty Spaces"',
+ ... l="'Void Space'"),
+ ... )
+ ... )
+ -BWSne+tBlank\040Space -Baf -F+t"Empty\040\040Spaces" -l'Void\040Space'
  """
  gmt_args = []
  # Exclude arguments that are None and False
@@ -168,11 +177,13 @@ def build_arg_string(kwargs):
  for key in filtered_kwargs:
  if is_nonstr_iter(kwargs[key]):
  for value in kwargs[key]:
- gmt_args.append(f"-{key}{value}")
+ _value = str(value).replace(" ", r"\040")
+ gmt_args.append(rf"-{key}{_value}")
  elif kwargs[key] is True:
  gmt_args.append(f"-{key}")
  else:
- gmt_args.append(f"-{key}{kwargs[key]}")
+ _value = str(kwargs[key]).replace(" ", r"\040")
+ gmt_args.append(rf"-{key}{_value}")
  return " ".join(sorted(gmt_args))